There are a subset of the airborne radar systems presently available that are capable of detecting and tracking ground moving vehicles at tactically significant speeds and over a wide geographic region. Depending on the mode of the radar (i.e., synthetic aperture radar imaging—SAR), individual human movement at a point or within a small area of interest can potentially be observed. However, these systems are incapable of tracking ultra slow targets that move within or through a large, rectilinear ground swath: tracking requiring detection at low false alarm rate, with the required level of measurement accuracy and at sufficient update rate to overcome ambiguous report-to-track assignment. The ground swath being significantly larger than a point or area of interest, for example a beam spot projected onto the ground.
There have been many radar systems, both military and commercial, developed and fielded to detect slower targets such as walking or running individuals. However, none have been executed from a moving platform, let alone one that is airborne. All such systems are designed and operate from fixed locations; even if this radar system is vehicle-borne it is still operated when the vehicle becomes motionless, e.g., NIDARS-E developed by Sensor Technologies & Systems, Inc. The radar system are operated from non-moving platforms for any of the following conditions: 1) the Doppler shift imposed on the clutter return due to the motion of the platform can obscure the return from the slowly translating target; 2) the complexity required to compensate for the platform motion raises the radar system costs out of the target marketplace; or 3) the mission application may dictate a fixed-site solution, i.e., perimeter coverage for a forward deployed base or fire base.
The detection of ground moving targets using airborne radar is an evolving research and development area. However, current systems using airborne radar have been focused on wide area search and tracking of slow moving military vehicles. Both mission requirements necessitate radar antenna having long aperture lengths; the first to provide accurate cross range estimates of the target position, and the second to limiting the dominant ground clutter return to the smallest realizable Doppler frequency region. Advanced signal processing algorithms then operate on the Doppler constrained clutter signals to essentially “arrest” the platform motion, thereby permitting detection of endoclutter targets, i.e., those with velocity components in the direction of the radar, along the radar line-of-sight, that lie within the mainbeam clutter Doppler return while presuming sufficiently low sidelobe levels. The resulting antenna sizes in order to detect, and accurately geolocate from standoff distances, the ground moving military vehicles can be up to on the order of 20 to 24 feet which, in turn, requires even larger transport class of aircraft to support the airborne antenna, resulting in travel at even higher ground speeds. The increased ground speed, though offset by the antenna size, can result in a degraded minimum detectable velocity unless design, fabrication and signal processing measures are taken. However, such airborne radar system would require an antenna that becomes too large to transport (depending on the air vehicle), is prohibitively expensive (when gauged against the anti-personnel mission), or is unable to detect slow moving targets including ultra slow moving targets such as an individual walking.
More recently, smaller apertures have been employed on unmanned air vehicles (UAV). The high altitude UAV is slightly slower platform, than the transport class aircraft required to carry the very long aperture, and the antenna length can be relaxed somewhat to meet minimum detectable velocity (MDV) requirements, though at the expense of position estimation accuracy and tracking performance. One such high altitude UAV is the RH-4A Global Hawk. However, these high altitude UAV's are unable to meet the MDV requirement for detecting ultra slow targets. Synthetic aperture radar may provide a system for detecting ultra slow targets.
Synthetic aperture radar (SAR) techniques have been developed that support detection of ground moving targets, including those with slower target speeds. For instance, single aperture imagery is used to detect the blurring of the larger ground vehicle, provided that the target is sufficiently strong radar cross section (RCS), as it migrates through the image pixels during the image formation time, such as a streak in the image, while ignoring the cross range displacement of the target due to Doppler effects. There are various signal processing variants on the SAR technique known to one skilled in the art. Another technique for the detection of ground moving targets is known as along track interferometry (ATI) and uses two antennas that produce two SAR images. The ATI antennas are on a single platform and are separated by a fairly large distance in the along-velocity direction. The images are differenced and focused, thereby being capable of providing a velocity map of the scene. As with SAR, the ATI detection requires that the target must be sufficiently bright against the focused clutter returns in order to detect the ground moving targets. A third technique, known as Arrested SAR, employs a variant of space-time adaptive processing (STAP) and at least two antennas separated in the along-velocity direction of the moving platform, but having a much shorter baseline than ATI system. Arrested SAR, in which is essentially an imaging mode, requires that the beam remains ground registered for the same dwell, as would be required for a SAR image system. The processor for these SAR types of systems then cancels the clutter return and produces an image of all non-stationary objects. The SAR approach evidences all the effects of moving targets translating through the imaged scene, including blurring and azimuth displacement, but will enhance the target to interference ratio as the unmoving clutter is cancelled down to some residue level (a function of the radar system and platform errors).
However, these SAR-based approaches are very inefficient for a wide area search over a ground swath, because of the amount of time the beam must dwell on a single ground position. Resultantly, SAR-based approaches are ill-suited for ground swath coverage applications. For example, when a SAR-based system is in the stripmap mode, i.e., the antenna beam is steered to 90 degrees to the velocity vector of the airborne platform. The processor produces imagery at the rate of platform translation, thereby providing “push broom” coverage over the scan area. Image resolution is derived from the antenna size in this “strip map” mode of operation. If detection occurs, the radar system may scan a radar beam backwards to confirm the detection, i.e., a squinted beam collection. However, while the radar is backwards scanning, the system necessarily must sacrifice coverage over some other region because the radar has then fallen behind the aircraft motion and cannot catch up. The SAR-based approach prohibits target tracking while maintaining coverage as the aircraft flies along.
Another difficulty with the above mentioned approach is the image resolution required for reliable detection. Typically, on the order of 5–10 pixels per characteristic target dimension are necessary to reliably detect in an image, with low false alarm rate. Point scatterers, by definition, detect only in a single pixel. Fence posts, lamp posts, street sign posts, etc. are all strong targets that are very small spatially. To meet the detectability resolution constraint, and be distinguishable from clutter discretes, the image resolution must be significantly expanded. The radar cross section, of clutter and targets, decreases as the resolution of the radar decreases. Significantly, as the RCS of a target object become sufficiently small when over-resolved, e.g., an adult human form having nominally 1 m2 under-resolved RCS, the signal to noise ratio (SINR), or signal to noise ratio (SNR) decreases. Additionally, the even longer aperture time ensure that there will be target blurring as the ultra-slow target migrates through range and azimuth cells during the aperture time associated with the higher image resolution.
Accordingly, it would be desirable to provide a system capable of detecting moving individuals moving within a geometric area, such as a rectilinear ground swath. It would also be of benefit to provide a system capable detecting ultra slow targets moving within the ground swath. Of further benefit would be a system that is capable of providing periodic surveillance to detect and track ultra slow targets, such as individuals, that were entering and crossing the ground swath. Moreover, it would be advantageous to provide a system capable of detecting the ultra slow targets moving within a ground swath during all weather, day and night operations. Further, it would be of benefit to provide such a system capable of covering very large areas, typically many times larger than optical systems, with short revisit times and higher repetitive surveillance rates consistent with developing a smoothed track estimate, thereby boosting the amount of information available on the target. This information includes higher order modulations induced from the relative movement of the parts of the body to permit discrimination of an individual from nuisance targets, such as animals. Another desired attribute would be to provide a system capable of surveillance from afar, thereby providing advantageously anti-tip-off coverage over a coverage area, and yielding the rapid response, flexible employment and access permitted by the airborne platform.